1. Field of the Invention
The present invention relates to a method for homogenizing the imaging of a body region produced from a magnetic resonance measurement, in particular a body region produced by signals from a local antenna of small dimensions, such as an endorectal coil, and an antenna arrangement that is arranged outside the body and that has larger dimensions than the local antenna, such as a body array antenna, in order to obtain a combined image having pixels that are formed from first signal amplitudes received by coil elements of the antenna arrangements and second signal amplitudes received by the local antenna.
2. Description of the Prior Art
Magnetic resonance tomography is a known technique for obtaining images of the interior of the body of a living object under examination. In order to carry out magnetic resonance tomography, a basic field magnetic generates a static relatively homogeneous basic magnetic field. During the recording of magnetic resonance images, rapidly switched gradient fields for spatial coding, which are generated by gradient coils, are superimposed on this basic magnetic field. Radio-frequency transmitting antennas are used to irradiate radio-frequency pulses into the object under examination in order to trigger magnetic resonance signals. The magnetic resonance signals caused by these Radio-frequency pulses are picked up by radio-frequency receiving antennas. The magnetic resonance images of the examined region of the object under examination are constructed up on the basis of these magnetic resonance signals received by the receiving antennas. Each pixel in the magnetic resonance image is assigned to a small body volume in this case. The brightness or intensity value of the pixel is dependent on the signal amplitude, received from this body volume, of the magnetic resonance signal. Whole-body radio-frequency antennas, surface antennas, in particular, local antennas of small dimensions that can be inserted into the body can be used as receiving and/or transmitting antennas. The local antennas that can be inserted into the body—as a rule via a catheter—receive the magnetic resonance signals from a comparatively small body region. Just like surface antennas, such local antennas have the advantage by comparison with whole-body radio-frequency antennas of a better signal-to-noise ratio. The signal-to-noise ratio of the local antennas that can be inserted into the body, in turn is substantially higher than that of a surface antenna, owing to their small dimensions. However, the sensitivity of such a local antenna is strongly inhomogeneous over the measuring volume. The intensity of the received signal decreases with the distance from the signal source, i.e., the atomic nuclei emitting the magnetic resonance signals. This is manifested in the magnetic resonance images by regions of the measuring volume that are situated near the local antennas appearing with a higher intensity than regions that are situated further away.
Because of its high signal-to-noise ratio, an endorectal coil is frequently used as local antenna in prostate examinations. An endorectal coil can be inserted directly into the body of the person to be examined and brought to the appropriate measuring point. Although a high signal-to-noise ratio is obtained in the vicinity of the prostate with the endorectal coil, the depth of penetration of the coil with regard to the reception sensitivity is limited to approximately the size of the coil. Consequently, in the case of such measurements, simultaneous use is made of a surface antenna that is formed from a number of coil elements and is a so-called body array coil or body array antenna, in order also to irradiate the surrounding tissue when measuring.
In the case of this imaging technique, in which a body array coil and a local antenna that can be inserted into the body are used for simultaneously receiving the magnetic resonance signals, the individual pixels of the magnetic resonance image are composed of signal amplitudes that have been received by coil elements of the body array coil and signal amplitudes that have been received by the local antenna. The strong difference in the signal-to-noise ratio between the two antenna systems leads to problems, however, in representing the image.
In order to achieve a maximum signal-to-noise ratio, the signal amplitudes of the individual coil elements of the antennas are combined as a rule using the following algorithm to obtain the intensity or brightness value G of a pixel:G=√{square root over (|E|2+|B1|2+|B2|2+ . . . +|Bn|2)},
Be denoting the signal amplitude received by the e-th element of the body array coil, and E denoting the signal amplitude received by the coil element of the local antenna.
Calculating the individual pixels by means of this algorithm yields the best achievable signal-to-noise ratio, and the background noise is constant over the entire magnetic resonance image. The sensitivity profile or the sensitivity distribution of this combined magnetic resonance image is, however, very inhomogeneous, because of the strong difference between the signal-to-noise ratios of the body array coil and the local antenna. This inhomogeneity can have a disturbing effect in particular in the case of image viewing.
In order to homogenize such a combined magnetic resonance image, conventionally an additional damping of the received signals has been produced by mismatching of the endorectal coil. Although this damping can reduce the difference in intensity between the signal amplitudes of the endorectal coil and body array coil, the high signal-to-noise ratio of the endorectal coil is reduced in the process.
German PS 195 26 778, corresponding to U.S. Pat. No. 5,712,567, discloses a method for compensating the sensitivity profile of an antenna arrangement that is visible in a magnetic resonance image, the method using two or more coil elements located opposite one another. The region of the object that is to be examined is located between the coil elements situated opposite one another. A magnetic resonance image of the same object region is picked up with the two coil elements or groups of coil elements in each case. By forming the average value from the intensity or brightness values of mutually corresponding pixels in the two magnetic resonance images, a geometrically averaged magnetic resonance image is formed, and a noise-reduced intermediate image is formed by combination. The noise-reduced intermediate image is corrected by means of a reception profile calculated from the averaged and the noise-reduced image, in order to obtain a homogeneous magnetic resonance image. Such a technique can be used, for example, to compensate the sensitivity distribution of a body array coil, but does not solve the problem of the different sensitivity distributions of the body array coil and local antenna that is able to be inserted into the body.